Change the color of the default (no) material (Cycles)
Open Blender, go to the Materials tab from the drop down select Material. Make your required changes, you code enable Nodes and do something really complex or just change the colour. Then go to the File menu and Save Startup File.


Now when you open Blender the default material will be your custom one.
You can also do this to have a default background.


Alternatively, you can set a single material for all objects from the Render Layers tab. Handy for Clay renders.